Early Apollo suits (like one with Thermal Micrometeoroid Garment [TMG] in the picture you referenced) all had a separate TMG. (Note the suit you referenced is most likely either an A2H or A2L with prototype TMG). The baseline concept (prior to the Apollo 1 Fire) was to don the TMG before the crew went outside for EVA. This is very similar to when we go outside in cold weather. We put on heavy over clothes to help protect us from the cold.
The TMG for the early suits consisted of separate trousers, jacket, over boots, over gloves and over visor assembly. The fidelity of these TMG components evolved through thermal testing to the version slated for the A6L Apollo suit. Had the Apollo 1 fire not occurred, this would have been what would have been worn on the lunar surface.
The outer cover of the A6L TMG was HT-1 a high temperature nylon, the same material as used on the Gemini suits (and Apollo A1C suit used on Apollo 1). While HT-1 was heat resistant, it was not fire proof.
As you know, one of the outcomes from the Apollo 1 fire tragedy was to make the spacesuits provide fire protection of the crew. Since the TMG multiple thermal layers already provided thermal protection, changing the outer cover layer to a fireproof material would make the TMG provide protection of the crew in the event of an Apollo 1 fire scenario. The TMG cover material was changed from HT-1 to fire proof Teflon coated beta yarn fabric, know as Beta cloth. The beta yarn (spun glass fiber) is fire proof and the Teflon coating sealed the fibers preventing glass fibers from breaking off and floating about in the zero-g environment. The TMG garment was integrated into the suit since it would be always needed (for fire protection inside the capsule and outside for EVA protection). The TMG designation changed to ITMG for Integrated TMG. The results of these changes was the A7L suit.
Since the TMG was integrated on the suit, it became a one-piece cover for the suit torso instead of separate pants and jacket. Over boots, separate EVA gloves and an over visor was still used to complete the TMG for EVA.
One of the limitations with Beta cloth is it is rather fragile. If sharply creased, the fibers will crack and eventually the fabric will split open where creased. For the Apollo A7LB suit, the outer cover layer of the TMG was changed to a more robust Teflon cloth made from Teflon yarn.
It should be noted that the horizontal pressure entry zipper of the A7LB can give the impression of a separate jacket and trouser configuration, but in reality the TMG is a one piece cover, as the horizontal pressure entry zipper does not go completely around the waist.
Also it should be noted that some A7L ITMGs were specifically made for training use. Since the Beta cloth was fragile, the ITMGs for these suits bore the designation of AT7L ITMG with the "T" in AT7L indicating "Training". The AT7L ITMG used a HT-1 outer cover layer instead of Beta cloth since the HT-1 was more robust for training use.